#class Array; def sum; inject( nil ) { |sum,x| sum ? sum+x : x }; end; end

module Math
  
  def tsum(a,b) 
    a + b - a * b
  end
  
#  def tsum(enum)
#    sum = 0
#    enum.collect { |e| sum =  sum + e - sum * e }
#    sum
#  end
  
  module_function :tsum
end